Contrary to popular belief, the three primary colors of light are red, green, and blue. The reason for this is that the rods in the back of your eyes detect red, green, and blue. If none of the cones fire up, than you see black. If say, only red fires up, than you see red, of course. When all of the cones fire up (red, green, and blue) than you see white. You know that optical illusion where you stare at a picture for thirty seconds and then look at a white sheet of paper and you see the same picture except the colors are inverted? This happens because the cones in the back of your eye get worn out from looking at the same colors for a long time. So when you look at a white sheet of paper, instead of all the cones firing, only the ones that weren't being used while looking at the image fire, so you see the opposite colors.